Oxygen isotopic record of silicate alteration in the Shergotty—Nakhla—Chassigny meteorite Lafayette

Abstract

Abstract— Samples from a suite of Shergotty—Nakhla—Chassigny (SNC) meteorites were analyzed for their O isotopic ratios by a modified version of the laser fluorination technique. Measured isotopic ratios ( 17 O/ 16 O and 18 O/ 16 O) from bulk samples of the Shergottites, EETA79001, Shergotty and Zagami; the Nakhlite Lafayette; and Chassigny are similar to those reported in the literature, as are those from olivine and pyroxene mineral separates from Lafayette. Iddingsite, a preterrestrial alteration product of Lafayette, was measured for the first time as a separate phase. Oxygen isotopic ratios increase with the percentage of iddingsite in a sample to a maximum δ 18 O of 14.4% for a ∼90% separate. Based on these measurements, end‐member iddingsite has a δ 18 O of 15.6%, which places it among other 18 O‐enriched secondary phases (carbonate and silica) observed in SNC meteorites. The relatively large difference in δ 18 O between iddingsite and the olivine and pyroxene it replaces (∼11%) is typical of low‐temperature alteration products. A range of crustal fluid δ 18 O values can be interpreted from the δ 18 O for end‐member iddingsite, assuming isotopic equilibrium was achieved during low‐temperature hydrous alteration (<100 °C; Treiman et al. , 1993). The calculated range of values, −15 to 5%, depends on many factors including: (1) the modal mineralogy of iddingsite, (2) potential isotopic exchange among other O‐bearing phases such as host silicate and carbonate, and (3) exchange with evolved or exotic O reservoirs on Mars. Despite the lack of constraints, the calculated range is consistent with isotopic exchange, and possibly equilibria, among components of the CO 2 ‐carbonate‐iddingsite‐H 2 O system at low temperature. The SNC meteorite samples in this study have Δ 17 O values that are indistinguishable from bulk Mars (0.30%), except for a single, small sample of iddingsite that has an anomalous Δ 17 O of ∼1.4%. While analytical difficulties make isotopic measurements for this sample problematic, the Δ 17 O is similar in direction to Δ 17 O reported for waters extracted from bulk samples of Lafayette (Karlsson et al. , 1992). If the Δ 17 O for iddingsite is confirmed, it can be concluded that evolved or exotic fluids on Mars have contributed volatiles to the O reservoir from which iddingsite formed 130 to 700 Ma ago.